Enable domain restricted sharing in an organization policy, and enable uniform bucket-level access on the Cloud Storage bucket.

Enable VPC Service Controls, create a perimeter around Projects A and B, and include the Cloud Storage API in the Service Perimeter configuration.

Enable Private Access in both Project A and B's networks with strict firewall rules that allow communication between the networks.

Enable VPC Peering between Project A and B's networks with strict firewall rules that allow communication between the networks.